Radio frequency transmitting apparatus having slotted metallic radio frequency windows

ABSTRACT

A slot antenna or radome having increased bandwidth capability may be made of a metallic sheet which is curved in the case of a radome, or flat in the case of the side of a waveguide, the sheet has a plurality of slots with edges which are beveled toward each other from the outer surface of the sheet, such that the metallic walls of the window are thin and approach the skin depth for the highest frequency of radio frequency waves which are to be transmitted. With longer wavelengths the electromagnetic field penetrates through the thin edge so that the window is effectively larger, thereby providing improved transmittance over a broad bandwidth.

United States Patent [191 Tricoles et a1.

[ RADIO FREQUENCY TRANSMITTING APPARATUS HAVING SLOTTED METALLIC RADIOFREQUENCY WINDOWS [75] Inventors: Gus P. Tricoles, San Diego; Eugene L.Rope, El Cajon, both of Calif.

[52] US. Cl. 343/770, 343/872 [51] Int. Cl. ..I-I01q 1/42 [58] Field ofSearch 343/770, 771, 872 R,

[56] References Cited UNITED STATES PATENTS 6/1968 Young 343/771 9/1954Risser 343/771 VIII/1V0,

Johnson 343/770 Martin 343/872 Primary Examiner-Eli Lieberman Attorney-Martin Lu Kacher [57] ABSTRACT A slot antenna or radome having increasedbandwidth capability may be made of a metallic sheet which is curved inthe case ofa radome, or flat in the case of the side of a waveguide, thesheet has a plurality of slots with edges which are beveled toward eachother from the outer surface of the sheet, such that the metallic wallsof the window are thin and approach the skin depth for the highestfrequency of radio frequency waves which are to be transmitted, Withlonger wavelengths the electromagnetic field penetrates through the thinedge so that the window is effectively larger. thereby providingimproved transmittance over a broad bandwidth.

11 Claims, 6 Drawing Figures RADIO FREQUENCY TRANSMITTING APPARATUSHAVING SLOTTEI) METALLIC RADIO FREQUENCY WINDOWS The present inventionrelates to apparatus for transmitting radio frequency waves andparticularly to radio frequency apparatus having slotted metallicwindows through which the waves are transmitted.

The invention is especially suited for use in slotted antennas andradomes and provides for improved transmittance characteristics over abroad band of radio frequencies.

Various types of microwave apparatus use windows through which microwaveenergy is transmitted. For example, slotted waveguides have been used asmicrowave antennas. It has also been suggested that radomes be made ofmetal and provided with slotted windows through which the radiofrequency waves can be transmitted. Metallic radomes and antennas areespecially desirable for use in aircraft since they provide a protectionfrom lightning and discharge static electricity accumulated duringflight. Metallic antennas and radomes are also helpful in reducing theradar cross section of the aircraft by causing divergent reflectionsfrom a streamlined surface, for example, of the radome. Such metallicradomes having windows have been described in an article by L. L. Oh, etal., entitled Fenestrated Metal Radomes which appeared in the MicrowaveJournal, Volume 7, page 62 (1964). Unfortunately, such radomes haveperformance drawbacks. They are particularly narrow-banded and have lowtransmittance.

It has been discovered, in accordance with the invention, that frequencydependence upon the size of the slotted window can be minimized bybeveling the edges of the slots so that the skin depth or thickness isapproached. For low frequencies the field penetrates into and throughthe beveled edges; for a wave at high frequencies, the skin effectprovides a slot of proper dimensions to support transmission of suchwaves, thus providing a slotted window which is virtually or effectivelylarger for radio frequency waves of lower frequency.

Accordingly, it is an object of the invention to provide improvedapparatus for transmitting radio frequency waves.

It is a further object of the invention to provide improved apparatushaving increased bandwidth for microwave radiation through slottedwindows.

It is a still further object of the present invention to provideradiating windows in the form of slots having increased effective slotsize with wavelength of the incidence wave.

It is a still further object of the present invention to provideimproved metallic, slotted antennas and radomes.

It is a still further object of the present invention to provide animproved radome having a slotted metallic surface which is transparentto radio frequency waves and can be used in place of conventionaldielectric radomes.

The inventon itself, both as to its organization and method ofoperation, as well as additional objects and advantages thereof, willbecome more readily apparent from a reading of the followingdescription, taken with the accompanying drawings, in which:

FIG. 1 is a side view of a radome embodying the invention; the viewbeing broken away to illustrate the wall of the radome in cross section;

FIG. 2 is a fragmentary sectional view of a slotted window in accordancewith an embodiment of the invention;

FIG. 3 is a sectional view ofa slotted metallic window which isconstructed in accordance with another embodiment of the invention;

FIG. 4 is a sectional view of a slotted window constructed in accordancewith still another embodiment of the invention;

FIG. 5 is a plan view of the slotted window shown in FIG. 4; and

FIG. 6 is a sectional view ofa slotted metallic window constructed inaccordance with still another embodiment of the invention.

FIG. 1 illustrates a fenestrated radome of streamlined shape which isconstructed from a sheet of metallic material, 10; the sheet suitablybeing aluminum, copper or stainless steel. The sheet, 10, has an arrayof slots 12 distributed uniformly in spaced relation over its entiresurface. In juxtaposition with the metallic sheet 10 is a thin sheet 14of dielectric material, such as fiberglass cloth which is laminated withepoxy resin. The metallic sheet 10 and the dielectric sheet 14 arejuxtaposed in side by side relationship. Preferably, the dielectricsheet is of a thickness much smaller than a wavelength of the radiofrequency (microwave) waves to be transmitted through the radome.

The slots 14 are rectangular in shape, say 1.5 centimeter long by 0.5centimeter wide for radio frequency waves of the order of 8.2 GHz.Opposite edges along the narrow sides of the rectangular are beveledfrom the 1.5 centimeter dimension which is mesured on the inner surface(interior of the radome), outwardly a distance of approximately 0.3inch. It has been found that the radio frequency waves of lowerfrequency, say from 7 GHz to 8.2 GHz, penetrate into and through thebeveled edges for greater thicknesses than the higher frequency waves,thus the slots are larger for the longer wavelengths and thetransmittance bandwidth of the radome is wider than for slots withoutthe beveled edges. It may be desirable to bevel the longer (1.5centimeter) edges of the rectangular slots in order to increase thetransmittance of the radome, particularly for larger scan angles. Thusthe radome would then be more transmissive for more incidence angles ofradiation, such angles being measured with respect to the axis of theradome.

FIGS. 2 through 6 illustrate tapered metallic windows which arefabricated in different ways. In each case the thickness will vary froma few ten thousandths to a few thousandths of an inch. Materials of themetallic sheet in which the windows are fabricated may be copper,aluminum or stainless steel. Steel is preferred where extremely thinedges are desired.

In FIG. 2 a metal sheet 16 is backed by a sheet 18 of dielectricmaterial, such as fiberglass cloth or ceramic material, the ceramicsilicon dioxide which is sold under the trade name Pryroceran by theComing Glass Company, of Corning, N.Y., is suitable. The slot 20 hasbeveled edges 22 which are formed by machining. It will be appreciated,of course, that the slot can be an aperture of any suitable geometricshape, for example, rectangular, oblong or circular and beveled on oneor more, or all, sides.

In FIG. 3, the metallic sheet 24 is backed by a thin layer of ceramicmaterial 26. The beveled edges 28 are provided by applying metallicpaint, such as a paint made by an emulsion of metal particles in anepoxy resin. The paint may be applied with a brush so as to form thelinearly tapered sides 28 of the slot.

FIGS. 4 and illustrate a slot 30 in a metallic sheet 32 which is backedby a sheet 34 of dielectric material. The beveled edges 36 of the slot30 is produced by photo chemical etching through several exposures andetchings. The material of the sheet 32 may be copper, having a thicknessof 0.007 centimeters. The six steps along the tapered edges 36 may be ofthe following thicknesses: step 38, 0.0022 cm; step 40, 0.0030 cm; step42, 0.0038 cm; step 44, 0.0046 cm; step 46, 0.0054 cm; and step 48,0.0062 cm. The area of each of the steps, 38 to 48, is the same. Thecentral opening, 50, may be 1.5 cm long by 0.5 cm wide.

FIG. 6 illustrates a slot 52 in a stainless steel sheet 54. The profileof the beveled edges 56 of the slot, which extends through the thicknessof the sheet, illustrated as being one thousandth of an inch, has acurved section 58, a section 60 of constant thickness, and a linearlytapered section 62 which extends to the opening 64 in the sheet 54. Thecurved section 58 is of a thickness approximately 0.005 inch or one-halfof the thickness of the sheet 54. This 0.005 thickness extends forapproximately one tenth of an inch. Then the linearly tapered section 62has a length of approximately twotenths of an inch. The overall lengthof the taper being approximately three-tenths of an inch. The profile asillustrated in FIG. 6 is especially suitable for stainless steel sheetswhich provide a broadbanded operation at microwave frequencies of theorder of 8 GHz.

From the foregoing description it will be apparent that there has beenprovided improved apparatus for transmitting radio frequency wavesthrough the use of slotted metallic windows. Such windows may findapplication in radomes, slotted antennas and other microwave aparatus.The variations and modifications in the herein described system willbecome apparent to those skilled in the art. Accordingly, the foregoingdescription should be taken as illustrative and not in any limitingsense.

What is claimed is:

1. An apparatus for transmitting radio frequency waves through ametallic member, a window through said member having a beveled edgewhich tapers at least to a thickness approaching the skin depth for thehighest frequency of said waves.

2. The invention as set forth in claim 1 wherein said member is ametallic sheet having a continuous, solid sheet of dielectric materialin side by side relationship therewith.

3. The invention as set forth in claim 1 wherein said member is a sheet,and said window is a rectangular opening in said sheet, opposite edgesof said rectangular opening are beveled to diverge in oppositedirections whereby said window is narrower on one surface of said sheetthan on the opposite surface thereof.

4. The invention as set forth in claim 3 wherein said beveled edgesextend to lines at the intersection of said beveled edges and the innerside of said sheet.

5. The invention as set forth in claim 3 wherein said beveled edges eachhave a plurality of steps, the steps at the narrowest dimension of saidwindow having said thickness approaching said skin depth for saidhighest frequency of said waves.

6. The invention as set forth in claim 3 wherein said beveled edges eachhave a profile including a curved section extending from the thickestportion of said edge a distance approximately one-half the thickness ofsaid sheet, followed by a section of constant thickness which isfollowed by a linear reduction in thickness to a point at the apex ofsaid edge.

7. The invention as set forth in claim 3 wherein said beveled edges eachare provided by a separate section of low-conductivity metallicmaterial.

8. A radome which comprises a sheet of metallic material, a plurality ofwindows which extend through said sheet, each window being formed by aslot having a beveled edge the bevel of which extends between theopposite surfaces of said sheet.

9. The invention as set forth in claim 8 wherein said slots are beveledon at least two opposite edges.

10. The invention as set forth in claim 9 wherein said edges are beveledinwardly toward each other in a direction away from the outer surface ofsaid radome.

11. The invention as set forth in claim 8 including a continuous sheetof dielectric material in juxtaposition with the surface of saidmetallic sheet which faces the interior of said radome.

1. An apparatus for transmitting radio frequency waves through ametallic member, a window through said member having a beveled edgewhich tapers at least to a thickness approaching the skin depth for thehighest frequency of said waves.
 2. The invention as set forth in claim1 wherein said member is a metallic sheet having a continuous, solidsheet of dielectric material in side by side relationship therewith. 3.The invention as set forth in claim 1 wherein said member is a sheet,and said window is a rectangular opening in said sheet, opposite edgesof said rectangular opening are beveled to diverge in oppositedirections whereby said window is narrower on one surface of said sheetthan on the opposite surface thereof.
 4. The invention as set forth inclaim 3 wherein said beveled edges extend to lines at the intersectionof said beveled edges and the inner side of said sheet.
 5. The inventionas set forth in claim 3 wherein said beveled edges each have a pluralityof steps, the steps at the narrowest dimension of said window havingsaid thickness approaching said skin depth for said highest frequency ofsaid waves.
 6. The invention as set forth in claim 3 wherein saidbeveled edges each have a profile including a curved section extendingfrom the thickest portion of said edge a distance approximately one-halfthe thickness of said sheet, followed by a section of constant thicknesswhich is followed by a linear reduction in thickness to a point at theapex of said edge.
 7. The invention as set forth in claim 3 wherein saidbeveled edges each are provided by a separate section oflow-conductivity metallic material.
 8. A radome which comprises a sheetof metallic material, a plurality of windows which extend through saidsheet, each window being formed by a slot having a beveled edge thebevel of which extends between the opposite surfaces of said sheet. 9.The invention as set forth in claim 8 wherein said slots are beveled onat least two opposite edges.
 10. The invention as set forth in claim 9wherein said edges are beveled inwardly toward each other in a directionaway from the outer surface of said radome.
 11. The invention as setforth in claim 8 including a continuous sheet of dielectric material injuxtaposition with the surface of said metallic sheet which faces theinterior of said radome.